In the discussion thread, we were discussing ways of assembling the blocks, I had an idea and then came up with version 1 of the block. I envisioned printing out a support structure for 3 pins on each plane to press fit into the 3.2mm holes spaced 5mm on center (per the Gadgeteer spec). This is the first design:

While this version was great on paper (well, in Sketchup), a few problems came about when I printed it out on my Makerbot Replicator (as seen below). Being that the only flat surface was that of the bottom, the object was printed in the orientation as shown, however due to the lack of support for the locating pins on the sides of the faces (not the top), as the plastic filament cooled, they drooped slightly and deformed, thus not being able to fit into the 3.2mm holes.

I also wanted to figure out a way to be able to use #4-40 bolts to affix the plates. I decided to make a few changes, such as making the figure a complete cube to allow threaded holes for the bolts to attach to, which also meant I had to worry about the depth of the bolts and hitting each other. I did some measuring and if I use 1/4” length #4-40 bolts, I had just enough depth with the addition of the plate thickness (1.6mm) that the bolts did not run into each other if screwed in on the front facing corners. That did mean though that the bolts on the top plate would be spaced further out. I decided to flip my idea and have the top bolt holes be those on a 45 degree plane from the corner and the side bolt holes be the in the second row of mounting holes. I added locating pins on the first row of each holes on the corner and made them slightly smaller to account of the drooping of the plastic during cooling.

This is what version 2 looks like in Sketchup:

And here are a few pics of version 2 actually printed out and mounted first, on two rectangular FBB plates, and then with a square plate as the lid:

And here are two blocks holding together 3 smaller rectangular plates and a full plate on the bottom.

I plan to make a few revisions to fix the offset problem with the edges of the FBBs.

I also plan to make a version of this to work with my custom mounting boards (available for sale soon), but they should also work with any other Gadgeteer spec boards, such as those from Nwazet or Tamiya. I also want to make the threaded holes slightly smaller as the holes on the top did not grip as well as those on the front faces (I think the droop actually helped in this instance)

I also plan to make a flat connector to that you can “butt weld” two plates together to extend the length if needed.